1. Field of the Invention
The present invention relates generally to a switching regulator, and is directed more particularly to a switching regulator in which the power consumption by a starting register for a pulse width modulator under the stationary state is reduced and hence the efficiency of the switching regulator is improved.
2. Description of the Prior Art
Generally speaking, in the prior art switching regulator, when a power supply switch is tuned ON, there may occur an excessive surge current flowing thereinto and hence the diodes thereof for rectification are damaged. To avoid this, as shown in FIG. 1, there has been proposed such a switching regulator in which, for example, a resistor is provided at the output side of a rectifying circuit for avoiding a surge current, the resistor is opened or closed by a change-over switch, and a pulse width modulator is supplied with a power through a starting resistor upon starting, while supplied with a DC voltage, which is derived therefrom, through a tertiary winding of a transformer and then smoothed, at the stationary state. That is, in FIG. 1, a power supply plug 1, which is supplied with a commercial power supply voltage, is connected through a power supply switch 2 and a power supply fuse 3 to a rectifier circuit 4 whose output side is connected through a surge current preventing resistor 5 and a primary winding 6a of a switching transformer 6 having a magnetic core to the collector of a switching transistor 7 whose emitter is grounded and whose base is connected to a pulse width modulator 8 to be supplied with a PWM (pulse width modulated) signal therefrom. A switch 9 is connected in parallel to the resistor 5. This switch 9 is opened to insert the resistor into the path upon the power supply switch 2 being closed, and which is closed to disconnect the resistor 5 from the path upon the stationary state being reached. The output terminal of the resistor 5 is grounded through a series connection of a starting resistor 10 and a capacitor 11. The connection point between the resistor 10 and the capacitor 11 is connected to one input terminal of the pulse width modulator 8 to apply a starting current thereto and hence to initiate the operation of the switching regulator. The above capacitor 11 serves to gradually increase the duty ratio of an output PWM signal from the pulse width modulator 8 upon the power supply being started by gradually increasing the voltage across the capacitor 11, so that this capacitor 11 is generally called a soft start capacitor. A rectifier circuit 12 is connected across a secondary winding 6b of the switching transformer 6 to provide a stable DC voltage for a load (not shown) in response to the switching operation of the switching transistor 7. A relay winding 13 is connected to the output side of the rectifier circuit 12 to be energized by the output from the rectifier circuit 12 and to open or close the changeover or relay switch 9. The switching transformer 6 has a tertiary winding 6c which is connected at its one end to the ground and at its other end to the input side of a rectifier circuit 14. A DC voltage appearing at the output side thereof is applied to the input terminal of the pulse width modulator 8 as a power voltage during the stationary state. The above other end of the tertiary winding 6c of the transformer 6 is also connected through a rectifier circuit 15 to a variable resistor 16 for adjusting a voltage applied from the rectifier circuit 15 to the pulse width modulator 8. At the output side of the variable resistor 16 there is obtained a compared voltage which corresponds to the DC voltage obtained at the output side of the rectifier circuit 12. This compared voltage is fed to the other input terminal of the pulse width modulator 8 to be compared with a reference voltage. Thus, the pulse width modulator 8 produces, based upon a compared error voltage, a PWM (pulse width modulated) signal which is applied to the base of the switching transistor 7 so that the ON-time thereof is controlled and the rectifier circuit 12 produces at its output side a stable DC voltage.
With a switching regulator constructed as above, the relay switch 9 is opened, the power supply switch 2 is closed, thereafter the switching transistor 7 is supplied with the PWM signal from the pulse width modulator 8 to carry out the switching operation, and then the stabilized DC output voltage is obtained at the output side of the rectifier circuit 12. After the electric power is applied to the switching regulator, the relay winding 13 is energized and hence the relay switch 9 is closed. Thus, the resistor 5 is short-circuited and hence any power consumption by the resistor 5 is avoided upon the stationary state being reached. In the prior art, however, the starting resistor 10 is always inserted in the power supply circuit to the pulse width modulator 8, so that the power consumption by this resistor 10 under the stationary state involves a problem, namely that the efficiency of the switching regulator is deteriorated much.